1. Field of the Invention
The present invention relates generally to optical head structures for use in recording or retrieving information on or from optical information storage media including but not limited to optical disks, and more particularly to an optical head employing a laser module with a plurality of built-in semiconductor laser chips. This invention also relates to optical information media recording/reproduction apparatus using such optical head.
2. Description of Related Art
In recent years, as optical media storage devices—such as optical information recording and reproduction apparatus including optical disk drives—are becoming important more and more in the manufacture of electronic equipment, the trend has been toward optical read/write devices exhibiting smaller size and reduced thickness along with a variety of functionalities.
For example, it has been required that a single small-size optical head assembly be used to record and/or play back information to/from optical disks of different types. These include recordable compact discs, also known as compact disc-recordable or “CD-R” (trade name), and digital versatile disks (DVDs)—previously digital video disks under the same abbreviation. The former is recordable and non-erasable optical storage media as popularized in the market whereas the latter is ultrahigh-density writable optical storage media as recently developed, with two layers on each side of a disk to store video and other data. Whereas laser beams adaptable for use in recording/retrieving data to/from CD-R media measure approximately 780 nanometers (nm) in wavelength, those for DVD record/playback are about 660 nm in wavelength. In view of such laser wavelength difference, CD-R/DVD compatible record/playback apparatus should be required to come with separate or independent laser light source units—that is, both a laser light source of about 780-nm wavelength and a 660-nm laser light source—situated within a single optical head assembly. Prior known multiple light-source small-size optical heads are disclosed in, for example, JP-A-10-261240 and JP-A-10-289468. The optical heads as taught thereby are designed so that a 780-nm wavelength semiconductor laser chip for CDs and a 660-nm semiconductor laser chip for DVDs are integrally accommodated in one unit, along with more than one optical detection element operatively associated therewith.
Generally, light beams of different light emission positions tend to pass through different locations of a lens system at different angles; obviously, in the above-noted optical heads also, laser beams emitted from two semiconductor laser chips behave to reach different positions of a focusing lens at different angles. With the prior art optical heads as taught by the above-identified Japanese publications, in order to attain an increased storage density of data on optical media during recording and reproduction, the 660-nm wavelength semiconductor laser chip for DVDs is disposed at a specified location on the optical axis of a lens system consisting essentially of a focusing lens and collimating lens while letting the 780-nm wavelength semiconductor laser chip for CDs be laid out at a selected location outside of the optical axis of such lens system. This multi-laser layout would result in that while a laser beam for use in DVD recording/playback may straightly progress to hit the focusing lens at right angles thereto so that any appreciable aberration will hardly take place at a laser spot focused on the surface of a DVD, a laser spot on a CD can experience unwanted aberration (in particular, coma aberration) because of the fact that a laser beam for CD record/playback attempts to diagonally falls onto the focusing lens at an angle thereto.
To avoid this problem, the optical head disclosed in JP-A-10-261240 is designed so that a holographic optical device (designated by reference numeral “25” in the disclosure thereof) is employed to bend or curve only the optical path of a CD-read/write laser beam thereby letting it reach the focusing lens at right angles thereto. Similar beam path correction is done in the optical head of JP-A-10-289468, by alternative use of an optical composer (denoted by numeral 30 therein) including a polarizating prism (birefringent plate) or holographic device.
Unfortunately, the prior art approaches are encountered with a problem which follows. The intended laser beam path adjustability does not come without requiring use of “special” holographic devices or polarizing prisms (birefringent plates) that offer expected functionality of bending only the optical path of a 780-nm CD read/write laser beam while permitting a 660-nm wavelength DVD read/write beam to be kept free from any influence therefrom. Use of such special optical components disadvantageously results in an increase in production costs of optical head units.
Another problem faced with the prior art lies in the lack of sufficient capability to achieve light weight and slim size of optical head structures, thus failing to fully meet demands for thickness-reduction or down-sizing of optical heads. More specifically, although not specifically set forth in the above-identified Japanese documents, it will readily occur to those skilled in the art that those optical components other than the focusing lens must be laid out in a plane parallel to the disk surface while at the same time requiring use of an additional mirror, called a “riseup” mirror, in order to guide beams to accurately hit the focusing lens. Moreover, in order to attain the intended recording of information on a target disk, a beam reshaping prism should also be required for efficient collection or focusing of rays of light emitted from a semiconductor laser with anisotropic optical intensity distribution into a light spot with isotropic optical intensity distribution. Hence, it is inevitable for achievement of the required optical head at low costs to reduce the requisite number of associative optical components while simultaneously reducing production costs thereof.